Device for Determining the Position of at Least One Structure on an Object, Use of an Illumination Apparatus with the Device and Use of Protective Gas with the Device

ABSTRACT

A device for determining the position of a structure ( 3 ) on an object ( 2 ) in relation to a coordinate system is disclosed. The object ( 2 ) is placed on a measuring table ( 20 ) which is movable in one plane ( 25   a ), wherein a block ( 25 ) defines the plane ( 25   a ). At least one optical arrangement ( 40, 50 ) is provided for transmitted light illumination and/or reflected light illumination. The optical arrangement ( 40, 50 ) comprises an illumination apparatus ( 41, 51 ) for reflected light illumination and/or transmitted light illumination and at least one first or second optical element ( 9   a,    9   b ), wherein at least part of the at least one optical element ( 9   a,    9   b ) extends into the space ( 110 ) between the block ( 25 ) and an optical system support ( 100 ). The block ( 25 ) and/or the optical system support ( 100 ) separates the illumination apparatus ( 41, 51 ) spatially from the plane ( 25   a ) in which the measuring table ( 20 ) is movable.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/015,437, filed on Jan. 16, 2008, which, in turn, claims priority toGerman Patent Application No. 10 2007 007 660.8 filed on Feb. 13, 2007,and German Patent Application No. 10 2007 049 133.8, filed on Oct. 11,2007, and which also claims the benefit under 35 U.S.C. 119(e) of U.S.Provisional Application No. 60/889,595, filed on Feb. 13, 2007, all ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a device for determining the positionof a structure on an object. In particular, the invention relates to adevice for determining the position of a structure on an object inrelation to a coordinate system. The object is placed on a measuringtable which is movable in one plane, wherein a block defines a plane inwhich the measuring table is movable. At least one laser interferometerfor determining a positional displacement of the measuring table in theplane is further provided. At least one optical arrangement is providedfor transmitted light illumination and/or reflected light illumination.

The invention further relates to the use of at least one illuminationapparatus with a device for determining the position of at least onestructure on an object.

The invention further relates to the use of protective gas with a devicefor determining the position of at least one structure on an object.

A measuring device for measuring structures on masks or substrates usedfor the production of semiconductors is known from the lecturemanuscript “Pattern Placement Metrology for Mask Making” by Dr. CarolaBläsing. The lecture was given on the occasion of the Semicon EducationProgram congress in Geneva on 31 Mar. 1998. This lecture manuscriptdiscloses the basis of a device for determining the positions ofstructures on a substrate. With regard to the details of the operationand the structure of a device of this type, reference should be made toFIG. 1 of this patent application, which illustrates the prior art.

In measuring equipment and devices of the prior art, optical sensingmethods are still favored, although the measuring accuracy required(currently in the region of a few nanometers) lies far beneath theresolution achievable with the light wavelength used (the spectralregion of the near UV). The advantage over devices that operate usingoptical measuring methods lies essentially in a less complex design andeasier operation compared with systems using other sensing systems, forexample, with X-rays or electron beams.

A measuring device for measuring structures on a transparent substrateis also disclosed by the published application DE 198 19 492. Themeasuring device comprises a reflected light illumination apparatus, animaging device and a detector device for imaging the structures on thesubstrate. The substrate is placed on a displaceable measuring tablewhich can be displaced perpendicularly to the optical axis. The positionof the measuring table is determined by interferometric means. Thedetector apparatus registers the edge profiles created by thestructures. Based on the profiles, the position of the edges of therespective structure can be deter-mined in relation to a fixedcoordinate system.

A device of this type is disclosed, for example, in DE 199 49 005, DE198 58 428, DE 101 06 699 and DE 10 2004 023 739. In all these prior artdocuments, a coordinate measuring machine is described with whichstructures on a substrate can be measured. The substrate is placed on ameasuring table which can be moved in the X-coordinate direction and inthe Y-coordinate direction. Suitable light sources are used forilluminating the substrate. The substrate can be illuminated either bytransmitted light and/or by reflected light. For imaging the illuminatedstructures, a measuring objective which is also arranged in thereflected light ray path is provided. The light collected by theobjective lens is directed to a detector which, in conjunction with acomputer, converts the received signals into digital values.

The structures on wafers or the masks used for exposure permit onlyextremely small tolerances. In order to check these structures, a veryhigh degree of measuring accuracy (currently in the nanometer range) isneeded. A method and a measuring device for determining the positions ofthese structures are disclosed in the German specification laid open toinspection DE 100 47 211 A1. For details of the positional determinationdescribed, reference is therefore expressly made to this document.

Previously, devices for measuring masks or structures on masks have usedmercury-xenon lamps for illuminating the measuring optical system. Thesehave a very marked intensity maximum in their spectrum at 365 nm. Thiswavelength or the region round this wavelength is used for illuminatingthe measuring optical system. The energy in this line has previouslybeen sufficient for illuminating the measuring optical system. In futuresystems, due to the increased demands placed on the resolving power, itwill be necessary to change over to ever shorter wavelengths (248 nm,193 nm, 157 nm). This higher resolution will be demanded by customerssince the structures on the masks are becoming ever smaller. However, atthese wavelengths, the lamps typically used for illumination inmicroscopes do not produce any spectral lines of sufficient intensity.It is therefore necessary to make use of alternative light sources oralternative configurations of the device for measuring structures on asubstrate. The necessary spectral lines are not present at sufficientintensity in the wavelength range required here.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a devicewith which it is possible to carry out examination of masks andsubstrates with smaller structures. In addition, the range within whichthe object to be measured is moved must not be influenced by heatproduction from possibly suitable illumination apparatus.

This object is solved with a device for determining the position of astructure on an object in relation to a coordinate system, the devicecomprises a measuring table carrying the object, wherein the measuringtable is movable in a plane, a block defines the plane, wherein at leastone laser interferometer system is used for determining a positionalchange of the measuring table in the plane, at least one opticalarrangement is provided for transmitted light illumination and/orreflected light illumination of the object, an illumination apparatusfor reflected light illumination and/or transmitted light illuminationand at least one optical element are provided, wherein at least one partof the at least one optical element extends into a space formed betweenthe block and an optical system support, wherein the block and/or theoptical system support spatially separates the illumination apparatusfrom the plane in which the measuring table is movable.

It is a further object of the invention to design an illuminationapparatus for use with a device for determining the position of at leastone structure on an object such that the device can be used to measureobjects with smaller structure separations.

The above object is solved by use of at least one illumination apparatusin a device for determining the position of at least one structure on anobject, wherein the at least one illumination apparatus is provided inthe reflected light illumination apparatus and/or the transmitted lightillumination apparatus, and that the illumination apparatus provideslight for a first optical element and/or light for a second opticalelement and that at least one system for triggering the illuminationlight is assigned to the illumination apparatus.

It is a further object of the invention to design a device for measuringstructures on objects such that the service life of the opticalcomponents is extended.

The above object is solved the use of protective gas in a device fordetermining the position of at least one structure on an object, whereinat least one optical component in the path of the light from at leastone illumination apparatus to at least one optical element is surroundedby protective gas.

When determining the position of a structure on an object in relation toa coordinate system, it is advantageous if the object is placed on ameasuring table that is movable in one plane. A block is provided whichdefines the plane in which the table can be moved. Furthermore, at leastone laser interferometer for determining the positional displacement ofthe measuring table in the plane is provided. At least one opticalarrangement is provided for transmitted light illumination and/orreflected light illumination. The optical arrangement also comprises anillumination apparatus for reflected light illumination and/ortransmitted light illumination of at least one optical element. At leastone part of the at least one optical element is provided in the spaceformed between the block and the optical system support. The blockand/or the optical system support separates the illumination apparatusfrom the plane in which the measuring table is movable.

The illumination apparatus comprises as the light source at least oneexcimer laser or at least one frequency multiplied solid-state laser orgas laser or at least one excimer lamp. The at least one optical elementwhich represents an objective lens is designed as a high-resolutionmicroscope objective which forms an image of the structure on thesurface of the object under reflected light and/or transmitted light inthe spectral range of the near UV on at least one detector.

There are several advantageous embodiments of the device with which theinvention can be realized. For example, the illumination apparatus ismounted only in the reflected light arrangement and the first opticalelement is mounted opposing the object in the reflected lightarrangement. In this embodiment, the first optical element is anobjective lens. A further possibility is that the illumination apparatusis only mounted in the transmitted light arrangement. The second opticalelement is then mounted under the object in the transmitted lightarrangement. The second optical element is a condenser. This arrangementcan also be regarded as a reflected light arrangement if the object isplaced in the measuring table such that the structures present on thesurface of the object face in the direction of the second opticalelement. In this orientation of the object, the second optical elementis also an objective lens (microscope objective). This arrangement hasthe advantage that the objects, masks or substrates are placed in thesame orientation in the device as the masks, objects or substrates areplaced when used in a stepper for the production of the semiconductors.

In a further advantageous embodiment of the device, the illuminationapparatus makes light available for reflected light illumination and fortransmitted light illumination. The first optical element is mounted asan objective lens opposite the object in the reflected light arrangementand the second optical element in the form of a condenser is mountedunder the object in the transmitted light arrangement. It is alsoconceivable for separate light sources to be provided for reflectedlight illumination and transmitted light illumination.

For the light source of the illumination apparatus, it is advantageousto use an excimer laser at a wavelength of 157 nm or 248 nm. Afrequency-multiplied solid-state laser or gas laser with a wavelength of266 nm, 213 nm or 193 nm can also be used as the light source for theillumination apparatus. An excimer lamp for the typical excimer laserlines can also be used.

The optical arrangement used with the device for measuring structures ona substrate can comprise in the illumination branch for reflected lightillumination and/or transmitted light illumination, respectively, atleast one apparatus for speckle reduction and/or at least one shutterand/or at least one homogenizer and/or at least one beam attenuator.

A possible arrangement of the various components of the opticalarrangement in the first illumination branch is that the illuminationapparatus has a beam attenuator connected downstream of it. Followingthe beam attenuator are the shutter, the apparatus for speckle reductionand the homogenizer. Once the light beam leaves the homogenizer, itreaches the first optical element. Furthermore, the illuminationapparatus can also have a beam monitor assigned to it. With the beammonitor, the intensity of the light emerging from the illuminationapparatus or the light source can be checked. Depending on the result ofthe checking, adjustment of the intensity of the illumination apparatuscan be carried out so that, finally, the same intensity al-ways falls onthe object to be measured.

A deflecting minor which directs the light from the illuminationapparatus in the first illumination branch through the optical systemsupport to the first optical element is provided. This is only the caseif the light from the illumination apparatus runs parallel to, and over,the optical system support. If the illumination apparatus with the beamattenuator, the shutter, the apparatus for speckle reduction and/or thehomogenizer is mounted under the block, that is, in the secondillumination branch, then again a deflecting mirror which directs thelight from the illumination apparatus through the block to the secondoptical element is also provided.

The illumination apparatus can also be arranged laterally on the device.Given a lateral arrangement of the illumination apparatus, the beamattenuator and the beam monitor can also be assigned to the illuminationapparatus. This lateral arrangement is advantageous because, for coolingthe illumination apparatus, an air stream can be directed unhinderedtowards the illumination apparatus and the additional components whichgenerate a substantial amount of heat. The object is to conduct away thedissipation heat in order that the heat generated does not influence thedevice and finally also the measuring results obtained with the device.

In an advantageous embodiment of the invention, one illuminationapparatus is provided. The light emerging from the illuminationapparatus is led or guided by suitable deflecting means or by dividerswhich divide the beam emerging from the illumination apparatus into thefirst illumination branch, which runs substantially parallel to theoptical system support, and into the second illumination branch, whichis provided under the block. In order to enable passage of the beamthrough the block, suitable perforations are provided in the block. Forthe event that the illumination branch runs parallel to, and over, theoptical system support, a suitable recess is provided in the opticalsystem support, which enables the passage of the illumination light.

The shutter used with the device can be configured as an obstructer oras a pivoting minor or as a movable divider or minor. A beam attenuatorcan be provided in the first or second illumination branch. The beamattenuator consists of a filter wheel on which plates having differenttransmittance values are arranged. According to need, the relevant platecan be moved by the filter wheel into the beam path of the first orsecond illumination branch. Furthermore, the plates can have differentreflection values. A further possible embodiment of the variable beamattenuator is that the angle of incidence of the light from the at leastone illumination source onto an inclined and coated substrate is varied.The attenuated light from the light source that is transmitted throughthe coated substrate can be further used in the device. The inclined andcoated substrate causes a beam offset. This beam offset can becompensated for by a further inclined substrate. The angular position ofthe individual substrates can be varied with motors.

The illumination apparatus for the reflected light or transmitted lightillumination has a homogenizer for the field illumination and/or ahomogenizer for the pupil illumination of the first optical elementand/or the second optical element.

The homogenizer can have different configurations. It can comprise aplurality of microlenses. It can also be configured as a hexagonal arrayof microlenses. An orthogonal array of microlenses is also conceivable.The microlenses can also be configured as a cylindrical lens array,wherein two crossed cylindrical lens arrays are provided. Themicrolenses can also have an aspherical surface. A further embodiment ofthe homogenizer is that a diffractive element is provided. Thehomogenizer can also consist of a light mixing rod.

An apparatus for speckle reduction can be provided in the firstillumination branch and/or in the second illumination branch. Thespeckle reduction apparatus can be diffractive in design. The apparatusfor speckle reduction can also be configured as a diffusion screen. Afurther design possibility for the apparatus for speckle reduction is amode mixing fiber.

The illumination apparatus is fastened to the device with a material oflow thermal conductivity in order to reduce the heat conduction to theoptical system support and/or to the block. In order to be able totransport away the dissipation heat effectively, cooling ribs are alsoprovided. As already mentioned, an air stream is directed towards theillumination apparatus in order to increase the effectiveness of theremoval of dissipation heat.

Advantageously, a climate chamber is provided, wherein the at least oneillumination apparatus is arranged outside the climate chamber. By thismeans, the influence of the dissipation heat generated by theillumination apparatus on the remaining components of the device issubstantially reduced. The climate chamber can be filled, for example,with a protective gas. Nitrogen has proved useful as a possibleprotective gas. The light from the illumination apparatus passes viawindows into the interior of the climate chamber.

A further advantageous embodiment of the invention is the use of atleast one illumination apparatus in a device for determining theposition of at least one structure on an object. The at least oneillumination apparatus may be provided in the reflected lightillumination apparatus and/or the transmitted light illuminationapparatus. The illumination apparatus provides light for a first opticalelement and/or light for a second optical element. The illuminationapparatus has at least one shutter assigned to it. As already mentioned,the illumination apparatus is provided with a light source whichcomprises at least one excimer laser or at least one frequencymultiplied solid-state or gas laser or at least one excimer lamp as theillumination source.

A further advantage of the invention is the use of protective gas in adevice for determining the position of at least one structure on anobject. At least one optical component in the path of the light from atleast one illumination apparatus to at least one optical element issurrounded by protective gas.

It is particularly advantageous if all the optical components in thepath of the light from the at least one illumination apparatus to theoptical elements are surrounded by protective gas. For this purpose, theoptical components are surrounded by an encapsulation and the light fromthe at least one illumination apparatus passes within the encapsulation.The protective gas in the encapsulation is nitrogen, since it isparticularly readily and economically available.

Further advantageous embodiments and uses of the invention are containedin the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the invention and their advantages will nowbe described in greater detail by reference to the accompanyingdrawings, in which:

FIG. 1 shows in schematic form a device for measuring structures on asubstrate, as has long been known from the prior art.

FIG. 2 shows an embodiment of the device, wherein the optical device isarranged together with the illumination apparatus over an optical systemsupport.

FIG. 3 shows a further configuration of the embodiment of FIG. 2,wherein the illumination apparatus also has a beam monitor assigned toit.

FIG. 4 shows an embodiment of the device, wherein the illuminationapparatus is arranged laterally on the device and wherein an air streamis directed onto the illumination apparatus.

FIG. 5 shows an embodiment of the invention, wherein the secondillumination branch is arranged under the block and wherein the lightfrom the illumination apparatus is directed onto the second opticalelement.

FIG. 6 shows an embodiment of the invention, wherein the illuminationapparatus also has a beam monitor assigned to it.

FIG. 7 shows an embodiment of the invention similar to the embodiment ofFIG. 6, wherein the illumination apparatus is mounted laterally on thedevice.

FIG. 8 shows an embodiment of the invention, wherein in the firstillumination branch and in the second illumination branch, in each case,an illumination apparatus is provided.

FIG. 9 a shows a substrate, which is placed on the table such that thestructures face in the direction towards the first optical element.

FIG. 9 b shows the substrate, which is placed on the table such that thestructures on the substrate face in the direction of the second opticalelement.

FIG. 10 shows an embodiment, wherein the illumination apparatus isprovided over the optical system support, and the light from theillumination apparatus is fed into the first illumination branch andinto the second illumination branch.

FIG. 11 shows a further embodiment of the invention, which differs fromthe embodiment of FIG. 10 in that the illumination apparatus is arrangedunder the block.

FIG. 12 shows an embodiment similar to the embodiment of FIG. 11,wherein the illumination apparatus is mounted laterally on the device.

FIG. 13 shows a further embodiment, wherein the illumination apparatusis also mounted laterally on the device, but the light from theillumination apparatus cannot be conducted through the optical systemsupport or the block into the first illumination branch or the secondillumination branch.

FIG. 14 shows an embodiment of the invention which is similar to theembodiment of FIG. 13, wherein the two outputs of the illuminationapparatus each have a shutter and a beam attenuator assigned to them.

FIG. 15 shows an embodiment, wherein the illumination apparatus is anexcimer laser.

FIG. 16 shows an embodiment of the illumination apparatus which is alsoconfigured as an excimer laser, wherein the excimer laser has a firstand a second output.

FIG. 17 shows an embodiment of the invention, wherein the device isarranged largely within a climate chamber.

FIG. 18 shows an embodiment, wherein all the optical parts of the firstillumination branch or of the second illumination branch are arrangedwithin an encapsulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic representation of a coordinate measuringmachine as has long been known from the prior art. The coordinatemeasuring machine is identified in the further description as a device.It should also be noted that in the description below and in thedrawings, the same elements are identified with the same referencesigns.

A device is used, for example, for determining the width (CD—criticaldimension) of a structure on a substrate 2. Also, using the device, theposition of a structure 3 on the substrate can be determined. Althoughthe device shown in FIG. 1 has long been known from the prior art, forthe sake of completeness, the operation of the device and thearrangement of the individual elements of the device will be discussed.

The device 1 comprises a measuring table 20, which is arrangeddisplaceable on air bearings 21 in a plane 25 a, in the X-coordinatedirection and in the Y-coordinate direction. For the mounting of themeasuring table 20, bearings other than air bearings can also be used.The plane 25 a is formed from one element 25. In a preferred embodiment,the element 25 is granite. However, to a person skilled in the art, itis obvious that the element 25 can be made from another material, whichprovides a precise plane for the displacement of the measuring table 20.The position of the measuring table is measured by means of at least onelaser interferometer 24 which, for the measurement, emits a light beam23 which hits the measuring table 20. The element 25 itself is mountedon oscillation dampers 26 in order to prevent building oscillationsreaching the device.

Placed on the measuring table 20 is a substrate 2, which bears thestructures to be measured 3. The substrate 2 can be illuminated with atransmitted light illumination apparatus 6 and/or a reflected lightillumination apparatus 14. The transmitted light illumination apparatus6 is provided in an optical arrangement 40. The reflected lightillumination apparatus 14 is also provided in an optical arrangement 50.The optical arrangement 50 comprises the transmitted light illuminationapparatus, a deflecting minor and a condenser. By means of thedeflecting mirror, the light from the transmitted light illuminationapparatus 6 is directed onto the condenser. The further opticalarrangement 50 comprises the reflected light illumination apparatus 14,a beam-splitting minor 12, the measuring objective 9 and a displacingdevice 15 assigned to the measuring objective 9. Using the displacingdevice 15, the measuring objective 9 can be displaced in theZ-coordinate direction (e.g. for focusing). The measuring objective 9collects light coming from the substrate 2 and deflects it out of thereflected light illumination axis 5 by means of the partiallytransparent deflecting mirror 12. The light passes to a camera 10 whichis provided with a detector 11. The detector 11 is linked to a computersystem 16 which generates digital images from the measurement valuesdetermined by the detector 11.

FIG. 2 shows an embodiment of the device 1 according to the invention.An optical arrangement 50 is arranged above an optical system support100. The optical arrangement 50 comprises at least one illuminationapparatus 51. In addition to the optical system support 100, a block 25is provided. The block 25 and the optical system support 100 arearranged such that they form an intermediate space 110. Provided in theintermediate space is a first optical element 9 a (objective lens). Thisfirst optical element 9 a is arranged opposing a measuring table 20which is arranged movable on the block 25 in a plane 25 a. The positionof the measuring table 20 is measured with at least one interferometer24 which directs a laser beam 23 towards the measuring table. Providedon the measuring table 20 is an object 2, in which the structurespresent on the object 2 can be measured with the first optical element 9a. The first optical element 9 a is arranged in a reflected lightillumination apparatus in relation to the object 2. The light from theillumination apparatus 51 passes via a deflecting mirror 60 to the firstoptical element 9 a. In the embodiment shown in FIG. 2, the light beamfrom the illumination apparatus runs parallel to, and over, the opticalsystem support 100. It is also conceivable, however, that the light beamfrom the illumination apparatus runs parallel to, and under, the opticalsystem support 100. In the embodiment shown in FIG. 2, the opticalsystem support 100 is provided with a recess 102 in order that the lightfrom the illumination apparatus 51 can pass unhindered to the firstoptical element 9 a. A camera 10 is provided for recording the imagesformed by the first optical element 9 a of the structures 3 on theobject 2. Furthermore, between the illumination apparatus 51 and thedeflecting minor 60, the optical arrangement 50 also has a beamattenuator 52, a shutter 53, an apparatus for speckle reduction 54and/or a homogenizer 55. In a particularly preferred embodiment, theillumination apparatus 51 is configured as an excimer laser. Theillumination apparatus 51 has, for this purpose, a first outlet 57 viawhich the light generated by the illumination apparatus 51 passes to thefirst illumination branch 200. Apart from the embodiment of theillumination apparatus 51 in the form of an excimer laser, furtherpromising alternatives for the design of the illumination apparatus 51are conceivable. One possibility for the design of the illuminationapparatus are so-called excimer lamps which emit light in the samewavelengths as excimer lasers. Furthermore, frequency-multipliedsolid-phase lasers and gas lasers can be used. Where, in the following,illumination apparatus and light sources are mentioned, the threepossible types of light source that can be used in the present inventionwith an expectation of success are always meant.

FIG. 3 shows another embodiment of the optical elements, which arearranged in the first optical arrangement 50 over the optical systemsupport 100. The construction of the device 1 shown in FIG. 3 isidentical to the construction of the device as per FIG. 2, except forthe beam monitor 56. The illumination apparatus 51 has a first outlet 58and a second outlet 59. Assigned to the second outlet 59 is a beammonitor 56 with which the quality of the light emitted by theillumination apparatus 51 can be monitored. It is thus possible with thebeam monitor 56 to determine intensity variations of the illuminationapparatus and to initiate a corresponding correction so that a constantintensity always falls on the substrate 2.

FIG. 4 shows an embodiment of the device 1 which is also essentiallyidentical to the configuration of the device according to FIG. 3. In thefollowing, not all the reference signs relating to the elements shown inthe drawings will be included so as to ensure the clarity of thedrawings and the associated description. In FIG. 4, the illuminationdevice 51 together with the beam attenuator 52 and the beam monitor 56are mounted laterally on the device 1. In the case illustrated here, theillumination apparatus 51 is provided laterally on the block 25. Thearrangement of the device laterally on the block 25 is only one ofseveral possible embodiments of the invention. The light emitted fromthe illumination apparatus 51 passes via the beam attenuator 52 to asecond deflecting minor 61. The deflecting mirror 61 is arranged suchthat it directs the light into the first illumination branch 200 of thefirst optical arrangement 50. The light is thereby deflected round theoptical system support 100 and only then passes, by way of the firstdeflecting minor 60, through the optical system support 100 to the firstoptical element 9 a. Due to the heat generated by the illuminationapparatus 51, it is useful to arrange it as far as possible from thesubstrate 2 to be measured. A particularly favourable arrangement isshown in FIG. 4. An air stream 70 can be directed towards theillumination apparatus 51 which is arranged laterally on the block 25,by which means the dissipation heat from the illumination apparatus 51can be removed particularly effectively.

FIG. 5 shows a further possible arrangement of the illuminationapparatus 41 in the device 1 according to the invention. Theillumination apparatus 41 is provided in the second optical arrangement40. The optical arrangement 40 is provided beneath the block 25 of thedevice 1. The light emitted from the illumination apparatus 41 reaches adeflecting mirror 62 and is thereby deflected to a second opticalelement 9 b (which functions here as an objective lens), which partiallyreaches into the space 110 between the block 25 and the optical systemsupport 100. The second optical element 9 a is arranged such that it isprovided opposite a substrate 2 which is laid on a measuring table 25.Furthermore, the second optical arrangement 40 can comprise a beamattenuator 42, a shutter 43, an apparatus for speckle reduction 44and/or a homogenizer 45. The deflecting minor 62 can also be constructedhalf-silvered so that the light coming from the substrate and capturedby the second optical element 9 a passes to a camera 10.

Depending on the orientation of the substrate on the measuring table 20,the embodiment of the invention shown in FIG. 1 or FIG. 5 can be usedboth in the transmitted light arrangement and in the reflected lightarrangement. The orientation of the substrate is intended to denotewhether the structures 3 present on the substrate 2 face in thedirection of the first or the second optical element 9 a or 9 b used forthe investigation, or whether the structures 3 present on the substrateface away from the first or second optical element 9 a or 9 b used forthe investigation. FIG. 9 a shows the substrate 2 in the conventionalorientation which means that the structures 3 on the surface of thesubstrate 2 face in the direction of the first or second optical element9 a or 9 b used for the investigation. If the substrate 2 is inserted inthe measuring table 20 with this orientation, then the arrangement inFIG. 1 is said to be a reflected light illumination arrangement. FIG. 9b shows the orientation of the substrate 2 in the measuring table 20wherein the structures 3 on the substrate 2 face away from the firstoptical element 9 a (in FIG. 1) used for the investigation. In contrastthereto, however, the structures 3 on the substrate 2 face toward thesecond optical element 9 b in FIG. 5. If the substrate 2 is inserted inthe measuring table 20 with the orientation shown in FIG. 9 b, theproposed arrangement of the first optical element 9 a as shown in FIG. 1is said to be a transmitted light illumination arrangement. With thearrangement of the second optical element 9 b as per FIG. 5, on theother hand, with the orientation of the substrate as proposed in FIG. 9b, it is said to be a reflected light illumination arrangement. Inaddition, the arrangement of the substrate 2 shown in FIGS. 9 a and 9 bshow that the substrate 2 experiences bending due to the support pointson the measuring table 20. The bending of the substrate 2 is representedin FIGS. 9 a and 9 b by solid lines and the bend substrate is identifiedwith the reference sign 2 d. The device as proposed in FIG. 5 isparticularly advantageous if the substrate with the orientation proposedin FIG. 9 b is inserted into the measuring table 20 with the arrangementproposed in FIG. 5. The arrangement proposed in FIG. 5 is thus used inthe reflected light arrangement. Therefore, with the arrangementproposed in FIG. 5, the substrates can be measured with the sameorientation as they have in a stepper. Added to this is the fact thatwith the apparatus as proposed in FIG. 5, the substrates are measuredwith the same wavelength as used in a stepper if the masks areilluminated on the wafer through the stepper.

FIG. 6 shows a further embodiment of the device as per FIG. 5, with thedifference that the illumination apparatus 41 also has a beam monitor 46assigned to it. The beam monitor 46 is assigned to the second outlet 49of the illumination apparatus 41. Thus the luminous power output by theillumination apparatus 41 can be monitored by the beam monitor 46.Depending on the measuring result from the beam monitor 46, theillumination apparatus 41 can be adjusted accordingly so that the sameintensity always falls on the object 2.

FIG. 7 shows a further embodiment of the device, in which at least theillumination apparatus 41 of the second optical arrangement 40 isarranged laterally on the block 25. The light from the illuminationapparatus 41 is guided with a deflecting minor 63 under the block 25 inthe second illumination branch 300. Otherwise, essentially all thecomponents of the optical arrangement 40 are identical to those in FIGS.5 and 6 and do not need further description here. In addition to theillumination apparatus 41, the beam attenuator 42 and the beam monitor46 can be provided laterally on the block 25. The illumination apparatus41, which is configured as a laser or as a conventional excimer lamp,causes heat generation. Through the arrangement of the illuminationapparatus 41 laterally on the block 25, it is possible for an air stream70 to be directed toward it to remove the dissipation heat of theillumination apparatus 41. It is obvious to a person skilled in the artthat the air stream 70 should be guided in suitable manner so that thedissipation heat is removed optimally. Turbulence caused by the airstream must also be screened off so that no other optical components ofthe device are influenced, as this would falsify the measurement valuesobtained in a non-reproducible manner. Mounting the illuminationapparatus 41 on the block 25 can be undertaken with suitable materials80. Suitable materials 80 have the property that they possess lowthermal conductivity. In order further to improve the removal ofdissipation heat, the material 80 may additionally be provided withcooling ribs (not shown). These cooling ribs naturally lie in the airstream 70 then.

FIG. 8 shows a further embodiment of the device, wherein in the firstillumination branch 200 and in the second illumination branch 300,respectively, an illumination device 51 and 41 is provided. Thus aseparate illumination apparatus 41 is provided for the reflected lightillumination arrangement of the first optical element 9 a (here theobjective lens). Similarly, for the transmitted light illumination withthe second optical element 9 b (here the condenser) a separateillumination apparatus 51 is provided. In the first illumination branch200, a shutter 53 is provided. A shutter 43 is also provided in thesecond illumination branch 300. The first shutter 53 and the secondshutter 43 are needed in the respective illumination branch 200, 300 inorder to switch between transmitted light and reflected lightillumination. If reflected light illumination is used or needed, theshutter 43 in the second illumination branch 300 is closed and viceversa. Whilst the measuring table 20 is moving and no images are beingrecorded, both shutters 53 and 43 are closed to reduce or avoid exposureof the mask or the object 2 to the beam. For this purpose the shutter53, 43 can be arranged at any position in the first illumination branch200 or in the second illumination branch 300. The arrangement of theshutter 43, 53 directly at the first outlet 48 or 58 of the firstillumination apparatus 51 or the second illumination apparatus 41 hasproved particularly favourable. This arrangement of the shutters 53, 43also reduces the illumination exposure of the various optical componentsin the first illumination branch 200 and/or in the second illuminationbranch 300, and this also increases their service life.

FIG. 10 shows an embodiment of the invention, in which the illuminationapparatus 51 is mounted above the optical system support 100. The deviceis configured such that with the device both the reflected lightillumination and the transmitted light illumination can be performed asdesired. A divider 65 is arranged in the first illumination branch 200.The divider 65 directs part of the light emerging from the illuminationapparatus 51 through the optical system support 100 and through theblock 25 to a deflecting mirror 63, which directs the illumination lightinto the second illumination branch 300. In order to guide theillumination light through for the second illumination branch 200appropriate recesses 106 and perforations 108 are provided in theoptical system support 100 and the block 25. As previously mentionedseveral times in the description, the light from the second illuminationbranch 300 is directed toward the second optical element 9 b(condenser). The light in the first illumination branch 200 is directedtoward the first optical element 9 a (objective lens).

The embodiment shown in FIG. 11 differs from that in FIG. 10 in that theillumination apparatus 41 is arranged under the block 25. The lightemitted from the illumination apparatus 41 into the second illuminationbranch 300 initially meets a divider 66. From the divider 66, part ofthe illumination light passes into the second illumination branch 200.The other part of the illumination light is deflected by the divider 66and passes through the perforations 108 and 106 in the block 25 and theoptical system support 100 to a deflecting minor 64 in the firstillumination branch 200. The light can thus be directed to the firstoptical element 9 a or the second optical element 9 b as desired. Asmentioned above, in the first illumination branch 200, a shutter 53 isprovided. Also in the second illumination branch 300, a shutter 43 isprovided. Depending on the choice of whether transmitted lightillumination or reflected light illumination is desired, the shutters 43or 53 can be actuated accordingly so that light is available in thefirst illumination branch 200 or in the second illumination branch 300.

As shown in FIGS. 10 and 11, arranged downstream of the firstillumination apparatus 51 is a beam attenuator 52 Likewise, arrangeddownstream of the second illumination apparatus 41 is a beam attenuator42. The beam attenuator 42, 52 serves to adapt the intensity to thereflection of the light source in order to avoid overdriving the camera10 in the imaging channel. In principle, the beam attenuator 52 or 42can be arranged anywhere in the illumination ray path 200 or 300. A solecondition for the arrangement of the beam attenuator 52 or 42 is that inthe first illumination branch 200 or in the second illumination branch300, the beam geometry must be suitable for the beam attenuator 52 or 42to be positioned at this site. In most beam attenuators, the attenuationdepends on the angle of incidence. Consequently, the beam attenuator 52or 42 is arranged at sites of small beam divergence. Particularlyadvantageous is an arrangement of the beam attenuator 52 or 42 directlybehind the shutter 53 or 43. This is advantageous since the opticalcomponents present in the rest of the first illumination branch 200 orsecond illumination branch 300 are exposed to a lower beam intensity.

FIG. 12 shows an embodiment, in which the illumination apparatus 41 isarranged laterally on the block 25. This arrangement of the illuminationapparatus 41 is essentially identical to the arrangement of theillumination apparatus 41 in FIG. 7. The light emerging from theillumination apparatus 41 is again fed into the first illuminationbranch 200 and the second illumination branch 300. For this purpose,again a divider 66 is provided which directs the light beam emergingfrom the illumination apparatus 41 through the recess 106 in the opticalsystem support and the perforation 108 in the block 25 to a deflectingminor 64, which then feeds the light into the first illumination path200.

FIG. 13 also shows the illumination apparatus 41 arranged laterally onthe block 25. The difference from the arrangement shown in FIG. 12 isthat the illumination apparatus 41 has a first outlet 48 and a secondoutlet 49. Arranged downstream of the first outlet 48 of theillumination apparatus 41 is a beam attenuator 42. Arranged downstreamof the second outlet 49 of the illumination apparatus 41 is a beamattenuator 52. The light from the illumination apparatus 41 coming fromthe first outlet 48 and the second outlet 49 is guided via a deflectingminor 63 or 64 into the first illumination branch 200 or into the secondillumination branch 300. Provided in both the first illumination branch200 and the second illumination branch 300 is a shutter 43 or 53. Withthe aid of the shutter 53, 43, the illumination can be controlled suchthat, according to wish, reflected light or transmitted lightillumination is provided.

FIG. 14 shows an embodiment of the invention, in which the illuminationapparatus 41 is also arranged laterally on the block 25. Arrangeddownstream of the first outlet of the illumination apparatus 41 is ashutter 43. Furthermore, a beam attenuator 42 is arranged downstream ofthe shutter 43. Also arranged downstream of the second outlet 49 of theillumination apparatus 41 is a shutter 53. Arranged downstream of theshutter 53 is also a beam attenuator 52. The illumination light for thefirst illumination branch 200 and the illumination light for the secondillumination branch 300 is fed laterally past the optical system support100 and laterally past the block 25 in this embodiment. The light fromthe illumination apparatus 41 is deflected by means of a deflectingminor 63 into the second illumination branch 300. The light from theillumination apparatus 41 which emerges from the second outlet 49 isdeflected by means of a divider 66 into the first illumination branch200. Part of the light passes from the divider 66 to a beam monitor 56with which, as mentioned several times above, the intensity of theillumination apparatus 41 can be monitored.

FIG. 15 shows an embodiment of the illumination apparatus 51. Althoughin the description below in relation to FIGS. 15 and 16, only thereference sign 51 is used for the illumination apparatus, it is obviousto a person skilled in the art that the same design conditions applyalso for the illumination apparatus with the reference sign 41. In FIG.15, arranged downstream of the illumination apparatus 51 is a shutter53. In the embodiment shown here, the shutter 53 is arranged directlydownstream of the first outlet 58 of the illumination apparatus 51. Inthe following description, the illumination apparatus 51 is a laser. Abeam attenuator 52 is arranged downstream of the shutter 53. The beamattenuator 52 has a first inclined plate 52 a and a second inclinedplate 52 b. The second inclined plate 52 b has the same quantitative,although opposite, angular position as the first inclined plate 52 a ofthe beam attenuator 52. The inclined plates 52 a and 52 b can beprovided, for example, with absorption filters in the known embodiments.A particularly advantageous embodiment is when the inclination angles ofthe individual plates 52 a and 52 b can be adjusted. Depending on thechosen angular position, a predetermined percentage of the light can bereflected out of the beam path. As already mentioned above, the beamoffset caused by the angled position of a plate can be compensated forby a second angled plate 52 b. If the angular position of the plates 52a and 52 b is driven by motor, the intensity level of the device can beset fully automatically.

FIG. 16 illustrates the same device as in FIG. 15 except that a beammonitor 56 is assigned to the second outlet 59 of the illuminationapparatus 51. The portion of the light 91 reflected out by the firstinclined plate 52 a passes to a beam trap 92 and is absorbed there. Thisalso generates dissipation heat which must not come near to thesubstrate or the mask. It is therefore advantageous if the beamattenuator 52 is arranged geometrically as far as possible from the maskand the substrate. As mentioned several times in the description of thedevice, the illumination apparatus 51 or 41 is arranged in an air streamso that the dissipation heat can be carried away. Since the beamattenuator 52 is also situated immediately following the first outlet 58or the second outlet 59 of the illumination apparatus 51, the beamattenuator is thus also arranged in the air stream, so that here too,sufficient cooling and the removal of dissipation heat can be carriedout.

FIG. 17 shows an embodiment of the device wherein the device 1 isarranged in a housing which is configured as a climate chamber 500. Theclimate chamber 500 is connected to a control system 501 so that thedesired pressure, humidity and protective gas environment can be set andmonitored. It might also be useful to conduct the light reflected out ofthe beam attenuator (see FIG. 16) out of the climate chamber. The beamtrap 91 can then be arranged outside the climate chamber. Thedissipation heat therefore no longer comes close to the substrate or theobject 2. It is also useful to arrange the illumination apparatus 41outside the climate chamber 500. The climate chamber 500 has suitablewindows 510 which are transparent for the wavelength of the light fromthe illumination apparatus 41, so that the light from the illuminationapparatus 41 passes into the interior of the climate chamber 500. In theembodiment shown here, the illumination apparatus 41 has a first outletand a second outlet. A shutter 53 and a beam attenuator 52 can bearranged at each of the two outlets. Part of the light from theillumination apparatus 41 passes from the divider 66 to a beam monitor56, by means of which, as mentioned several times above, the intensityof the illumination apparatus 41 can be monitored. From the divider 66,the light from the illumination apparatus 41 also passes into the firstillumination branch 200. The light from the illumination apparatus 41can be deflected by means of a deflecting minor 63 into the secondillumination branch 300. It is obvious to a person skilled in the artthat the illustration shown in FIG. 17 is not a limitation of theinvention. What is important here is only that as many of the componentsof the device as possible which produce dissipation heat should bearranged outside the housing. An air stream 70 for carrying away thedissipation heat from the illumination apparatus 41 and other componentswhich produce dissipation heat is directed towards these. It is obviousto a person skilled in the art that the air stream 70 should be guidedin suitable manner so that it produces optimum removal of thedissipation heat.

FIG. 18 shows an embodiment of the device, in which the overall ray pathof the light from the illumination apparatus inside and outside theclimate chamber 500 is additionally provided with an encapsulation 50 a.The encapsulation 50 a may be filled with a suitable protective gas froma reservoir 400. Nitrogen has proved to be a particularly preferableprotective gas. The use of protective gas is advantageous if for theillumination of the object 2 a wavelength is chosen that is smaller than220 nm. At this wavelength, the level of absorption in the normalambient air is too high. The cause of this is mainly atmosphericmoisture. In order to keep losses small, flushing out with protectivegas is therefore necessary. Many dry, inert gases are suitable asprotective gases. As previously mentioned, the use of nitrogen isparticularly advantageous since it is inexpensive and safe to use. Inaddition, hydrocarbons are always present in the normal ambient air.Light of these short wavelengths breaks the hydrocarbons down and theresulting decomposition products become deposited as a film on theindividual optical elements of the first optical branch and of thesecond optical branch. As a result of the deposition of thedecomposition products on the optical components, the transmissionproperties of these optical components become degraded. By means of theprotective gas flushing, therefore, this contamination by hydrocarbonson the surfaces is avoided and the service life of the opticalcomponents is extended. In the embodiment shown here, the illuminationapparatus 41, a shutter 43 and a beam attenuator 42 are provided outsidethe climate chamber 500. The shutter 43 is useful since with it thelight from the illumination apparatus 41 can be kept away from theremainder of the device when no measurement is being carried out withthe device. All the optical components of the device are therebyprotected from unnecessary exposure to the beam, thereby extending theirservice life. The light from the illumination apparatus 41 passes via awindow 510 into the portion of the encapsulation 50 a, which is situatedin the interior of the climate chamber 500. Part of the light from theillumination apparatus 41 is guided via a divider 66 parallel to theoptical system support 100. Although in the representation shown here,the light from the illumination apparatus 41 is guided above the opticalsystem support 100, this should not be regarded as a limitation of theinvention. From the divider 66, part of the light passes to a deflectingminor which deflects the light such that it is guided parallel to, andunder, the block 25. Provided in the light beam which passes parallel tothe optical system support 100 and parallel to, and under, the block 25,in each case, are a shutter 53, an apparatus for speckle reduction 54and a homogenizer 55.

As described above, the optical arrangement 40 or 50 can also comprise ahomogenizer 55 or 45. The homogenizer 55 or 45 serves to illuminate theobject field and the pupil evenly. The even object illumination ensuresthat the measuring result does not depend on the location of thestructure 3 being measured within the object field. Uneven pupilillumination leads to systematic measuring errors, which depend on theactual size of the structure 3. To avoid this, in critical applications,as in the measurement of the positions of structures 3 on an object 2,the pupil is homogenized.

If a laser is used as the illumination apparatus 51 or 41, the level ofcoherence of this light source is too high and speckles occur. Thisleads to a flecked and very noisy image and is not suitable to be usedfor the measurement of positions of structures 3 on an object 2. Duringevaluation, speckles of this type lead to errors in the positionaldetermination. In order to avoid this, it is necessary to use anapparatus for speckle reduction 54 or 44. These apparatuses areessentially based thereon that averaging is carried out over a pluralityof images, thereby ensuring that the speckles are not constant overtime. This can be done by one of the following methods.

If a pulsed light source is used, then the speckle pattern changesbetween two pulses. It is possible therefore to average over a pluralityof individual images. With continuous light sources, rotating groundglass disks suggest themselves. The averaging then takes place withinthe exposure time. It is also conceivable to use a glass fibre with modemixing properties. Averaging can then be achieved using these glassfibres.

The illumination apparatus 51 or 41 (except the excimer lamp) are pulsedlight sources. With these, inevitably variations in the intensity occurfrom pulse to pulse. In order to detect large anomalies or to be able tocorrect the actual pulse energy, it must be recorded together with themeasurements. Advantageous for this is the arrangement of a beam monitor56 directly behind the beam attenuator 52. The measuring result from thebeam monitor 56 can thus be used for automatic setting of the beamattenuator 52.

Also advantageous is the detection of the intensity before the firstoptical element 9 a (objective lens in the reflected light case) orbefore the second optical element 9 b (condenser in the transmittedlight case), since at this point, losses in the optical path to thispoint are detected. With progressive degradation of the opticalcomponents, the results from intensity measurements made directly in thevicinity of the illumination apparatus 41 or directly after the beamattenuator 42 no longer match the intensity that finally reaches theobject 2 or the mask. This would also lead to false results in themeasurement of the position of the structure. The use of the measuredintensity to correct the results when measuring the position ofstructures 3 on an object 2 and for determining the degradation of theoptical system is therefore advantageous.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. The use of protective gas in a device for determining the position ofat least one structure on an object, wherein at least one opticalcomponent in a path of the light from at least one illuminationapparatus to at least one optical element is surrounded by protectivegas.
 2. The use according to claim 1, wherein all the optical componentsin the path of the light from the at least one illumination apparatus tothe optical elements are surrounded by protective gas, wherein, for thispurpose, the optical components are surrounded by an encapsulation andthe light from the at least one illumination apparatus runs within theencapsulation.
 3. The use according to claim 1, characterized in thatthe optical components comprise at least one shutter, at least one beamattenuator, at least one apparatus for speckle reduction and/or at leastone homogeniser, as well as at least one optical element.